Producing aluminum coated metal articles



Oct. 30, 1951 G STERN 2,573,229

PRODUCING ALUMINUM COATED METAL ARTICLES Filed April 22. 1948 INVENTOR. 650 265 5752M BY /%%K A 7'7'OP/VEY Patented Oct. 30, 1951 PRODUCING ALUMINUM COATED METAL ARTICLES George Stern, Yonkers, N. Y., assignor to American Electro Metal Corporation, Yonkers, N. Y., a corporation of Maryland Application April 22, 1948, Serial No. 22,649

3 Claims.

This invention relates to the manufacture of shaped bodies mainly of ferrous material infiltrated with a cuprous infiltrant, for use under corroding conditions at elevated temperature but below its softening or melting point. More specifically the invention relates to the production of parts having suflicient strength and high damping capacity for jet propulsion engines and equipment, such as compressor blades and the like parts which are used in oxidizing gases. These bodies or parts comprise corrodible material and are therefore provided with a corrosion resistant coating.

Shaped bodies and parts of the above type have been prepared heretofore in a combined powder metallurgical and infiltration process by molding ferrous powder preferably of an average particle size of about 80 to 325 mesh under pressures of about to 30 tons per square inch (t. s. i.) whereby a shape having a density of about 65 to 75% was obtained. The porous shape was sintered thereafter, preferably at about 900 to 1150 C. for about one half to one hour in a protective or reducing atmosphere, subsequently reshaped or sized under pressure (coined) to a density of about '75 to 85%, and then infiltrated with copper or copper alloy. The infiltrated body may have required further sizing or shaping by coining if some warpage or other deformation occurred during infiltration, and was thereafter provided with a corrosion resistant coating of aluminum which was alloyed to the surface of the body and which formed its own corrosion resistant outer oxide surface.

When bodies and particularly parts of the type herein concerned have been required to exhibit a minimum yield strength of about 80,000 pounds per square inch (p. s. i.), appreciable amounts of carbon have been added to the initial ferrous powder so that the sintered shape contained up to 1.7% and preferably between about 0.1 and 0.8% carbon. After infiltration of the cuprous infiltrant, the infiltrated shape has been coined and then subjected to a heat treatment including a solution or diffusion heat treatment followed by cooling or quenching and back drawing. The corrosion resistant aluminum coating was applied thereafter preferably by dipping the heat treated body in molten aluminum or aluminum alloy.

It has been found, however, that such a heat treated body or part exhibits reduced physical properties after having been coated by dipping into molten aluminum and that initial properties could not be restored by subsequent heat treatments because the aluminum coating liquefies and fiows off at the temperatures of the heat treatment. If the coating of aluminum-bearing metal was applied to the heat treated infiltrated body or part by any other method than dipping, such as spraying, electroplating or otherwise, its bond with the body or shape had to be produced by an alloying heat treatment of the coated body or part.

According to the invention, the heat treatment of a body or part, mainly consisting of a ferrous skeleton containing combined carbon and infiltrated with a cuprous infiltrant, is combined with the aluminum coating process. The infiltrated body or part is dipped into molten aluminum or aluminum alloy at a temperature above the melting temperature of the aluminum coating and within the austenitic range of the ferrous skeleton whereby the infiltrated body is diffusion or solution heat treated and the aluminum coating is simultaneously applied thereto. The coated body is then cooled below the transformation temperature of the ferrous skeleton and the melting point of the coating and thereafter heat treated at a temperature level below that transformation temperature and the melting point of the coating. The ferrous skeleton is thereby hardened by precipitation of copper alloy and, if martensite is present in the skeleton, simultaneously softened by back-drawing. This heat treatment also precipitation hardens the infiltrant. Furthermore, the aluminum-bearing coating metal, which had little solubility in the ferrous skeleton in its austenitic state during dipping, is alloyed with the alpha iron of the skeleton and also with the cuprous phase of the body during this heat treatment.

It is therefore an object of the invention to produce an aluminum coated body or part of enhanced physical properties in a simple and economical manner.

It is another object of the invention to combine steps of the heat treatment of ferrous, infiltrated bodies or parts of the type herein concerned, with steps of a coating process for providing the body or shape with an aluminumbearing coating alloyed with surface metal of the body or shape.

It is a further object of the invention to produce bodies or shapes, such as parts of jet propulsion equipment, by a combined powder metallurgical and infiltration process from ferrous initial powder, carbon and a cuprous infiltrant, and to coat it with aluminum or aluminum alloy in a process which effects a heat treatment of the infiltrated body and which alloys its surface metal with the coating metal.

It is still a further object of the invention to provide shaped bodies of the type herein concerned with a coating including a substantially pure aluminum layer which can act as a dielubricant during pressure-shaping or sizing of the coated body subsequent to the heat treatment.

It is still another object of the invention to provide shaped bodies, and particularly parts for jet propulsion equipment for use at elevated temperatures but below about 500 C., of the type herein concerned.

These and other objects of the invention will be more clearly understood as the description proceeds with reference to the drawing in which Fig. 1 shows by way of exemplification a blade for jet propulsion equipment in elevation and partly in section. and Fig. 2 in plan view.

Referring to the drawing, the blade I 3 comprises a vane or air foil l and a root l I by which it is assembled. The blade [3 consists of a ferrous matrix permeated by a cuprous network,

and the coating comprises an alloy layer l4, an

aluminum layer l5 and a corrosion resistant outer surface layer IS.

The blade I3 is preferably produced by compacting ferrous powder under a pressure of about to t. s. i. and advantageously about 25 t. s. i. The ferrous powder consists of iron having an average particle size corresponding to below 100 mesh. If desired, the ferrous powder can be mixed with proper and known amounts of nowdery alloying constituents of alloy steel. such as. for instance, chromium, manganese, tungsten. tantalum. vanadium, titanium. which are known as carbide-formers. silicon. aluminum. co per. nickel, cobalt, which are known to have less tendency than iron to combine as carbide in steel. Any number of these elements can be added to obtain from the ferrous powder, upon shaping and sintering, a skeleton or matrix exhibiting the potentialities of alloy steel. Carbon is added or present in the skeleton in amounts up to 1.7%: however. if any alloying constituent is added to the powder or alloyed therewith in the subsequent manufacture, the amount of carbon should not exceed about 0.8% and preferably should be from 0.1 to 0.25%.

The compacted ferrous powder forms a coherent shape of a density preferably of to and is sinterecT at about 1100 C. to 1150 C. for about one hour in a protective atmosphere such as cracked ammonia. The sintered, porous bod is then sized or coined under a pressure of preferably about 30 to 35 t. s. i.: if a blade of the shape illustrated in the drawing is to be made. this coining can be combined with spreading the vane or air foil laterally to form the rounded leading and thin trailing edges l1, l8, respec tively. The coined body or blade now has a density of about to and is ready for infiltration. It is preferred to use an infiltrant consisting of about copper, 2% iron and 8% manganese. However, the proportions of the alloying elements of the copper can vary and in particular the iron can be omitted. Other alloying elements can also be added, particularly for the purpose of being incorporated by diffusion, at least in part, into the matrix. For this purpose, any alloying element as previously mentioned for addition to the ferrous powder can be used, provided that its admixture with the copper does not increase its melting point above about 1200 C.;

thus the addition of tungsten, tantalum and molybdenum is usually excluded, and the proportions of the other additions are accordingly limited as may be established from the phase diagrams of the respective systems.

The infiltration is accomplished by heating a preferably pressed-to-shape powdery mixture of predominant amount of copper and the alloying elements, or of copper alone, to a temperature of about 1l50 to 1250 C.; thereby the copper melts and dissolves the other elements. There can also be used a preformed alloy of copper and the desired additions which is molten at the temperatures stated. The mixture or alloy is preferably superheated at the infiltration temperature exemplified. Dependent on the liquidity of the molten infiltrant and the dimensions of the skeleton or porous matrix, infiltration may be completed within a few seconds or minutes. It is preferred to continue the heating of the infiltrated skeleton at about the infiltration temperature, up to hour and sometimes up to 1 /2 to 2 hours, in order to secure diffusion of a portion of the infiltrant into the skeleton to the limit of the solubility of the constituents of the infiltrant in the ferrous matrix.

More specifically, if a cuprous infiltrant of about 90% copper, 2% iron, and 8% manganese is used, and considering that the ferrous matrix at the temperature of infiltration is within its austenitic range, up to 8% and even 13% copper by weight of the matrix, and up to almost half of the manganese contained in the cuprous infiltrant can diffuse into the ferrous matrix. Of course, if copper and manganese were present in the ferrous skeleton before infiltration, the amount of copper and manganese which can diffuse from the infiltrant is reduced accordingly. The shorter the time period for which the infiltrated body is held at the infiltration temperature, the less are the amounts of metal constituents which diffuse from the infiltrant into the ferrous matrix or skeleton. On the other hand, constituents of the matrix also diffuse into or dissolve in the molten infiltrant if heating is continued at about the infiltration temperature, and their maximum amounts which can diffuse into the infiltrant depends on the limits of their solubility therein and the time period of continued heating. For instance, if the cuprous infiltrant contains less iron than is soluble therein at the infiltration temperature, and particularly if only 2% or none at all were added, additional iron can be dissolved in the cuprous infiltrant up to a total of 8% and more of its weight.

The infiltrated body is cooled thereafter at such rate as to retain copper dissolved in the ferrous matrix in excess of the amount which the matrix can hold at room temperature, and also to retain iron in the infiltrated copper phase in excess of the amount which it can hold in solution at room temperature. The rates of cooling for obtaining this effect are known and include quenching, air and furnace cooling. On quenching, the ferrous skeleton remains supersaturated with copper dissolved therein which tends to keep the matrix relatively soft, whereas substantially the structure of martensite is imparted to the iron-carbon system of the skeleton which tends to harden it. At a rate of cooling slower than quenching, some or even all of the carbon may form bainite or pearlite while some excess copper still remains in solution in the matrix. If manganese is present in the ferrous matrix (skeleton), a portion of it is apt to form ternary solid solutions with the copper and iron, while another portion is apt to form complex carbides with the carbon and iron. The cuprous infiltrant in which iron has been dissolved to the limits previously stated and in which also manganese and other alloying constituents, if present, may have been dissolved, retains them in solution upon quenching or gives up a portion of them upon cooling at a slower rate if the solubility of these constituents in copper is smaller at the lower temperature; for instance, the solubility of iron in copper at a temperature below the transformation temperature of the ferrous skeleton is reduced to less than 1% of the latter. In any event, upon quenching or cooling at controlled rate, the resulting body will be soft enough for subsequent shaping or sizing (coining).

After such shaping or sizing, if desired, the infiltrated body is coated with aluminum or aluminum alloy preferably containing about 4% copper and 0.5% manganese. The coating metal is melted and heated to a temperature substantially above the melting point, normally to a temperature of about 735 to 820 C. and oxidation in open air is prevented by a cover preferably consisting of a eutectic mixture of about 66.7% potassium chloride and 33.7% sodium chloride which melts below the temperature range stated.

Before the infiltrated and, if desired coined 11 body is dipped into the molten coating metal, it is cleaned preferably by passing it through a molten flux obtained from a mixture of '75 to 85% sodium and potassium chloride with 15 to cryolite which melts at about 700 to 750 C. and is superheated to about 800 to 850 C. The body remains in the flux for a period of time sumcient to heat it to the temperature of the flux; 20 to 40 seconds sumce for this effect in the case of a blade of average size.

The body or blade is then immediately transferred into the coating bath so that it is neither contaminated nor appreciably cooled. The time of dipping a body is preferably shorter than the time during which it was kept in the flux, e. g. 10 to seconds. The ferrous skeleton of the body is thus heated to a temperature within its austenitic range, particularly if its carbon content does not exceed about 0.8%, and only up to about 1% aluminum can alloy with the gamma iron. Therefore only slight alloying between the aluminum and the surface metal of the body occurs, but molten aluminum adheres to the surface of the body when it is removed from the bath. While the body was passing through the cleaning flux and dipping bath, it was diffusion or solution heat treated to some extent; this can be important in cases where the body has been cooled immediately after completion of infiltration and not kept at about the infiltration temperature for an additional period of time, and the time period of holding the body in the hot flux may be suitably extended for this purpose.

Excess molten aluminum is shaken off the body removed from the coating bath, and the body and coating thereon are allowed to coolto a temperature below the transformation temperature of the ferrous skeleton and the freezing point of the adhering aluminum-bearing coating, preferably to between about 300 and 600 C. While it is possible to cool the coated body preferably rapidly to room temperature and then to reheat it, it is preferable to cool it only to the temperature of the desired heat treatment, between 300 and 600 as it saves reheating. Quenching or air cooling are satisfactory.

The coated body is held within the last temperature range stated and preferably at 500 C. for about one hour. Thereby excess copper still in solution in the ferrous skeleton is precipitated, mostly as a copper alloy containing less than 1% iron and some manganese, if it was present in the skeleton. Thereby a hardening effect is obtained. If the ferrous skeleton has been quenched to form martensite, the reheating results in softening of the iron-carbon system in the skeleton. If the ferrous skeleton has been slowly cooled from the infiltration temperature so that its carbon-iron system formed pearlite or bainite, the reheating has no substantial effect upon it. It is understood that byproperly controlling the cooling rate from the infiltration temperature, followed by back drawing at a temperature between 300 and 600 C. and preferably about 500 C., the concurrent softening and hardening effects can also be controlled. Upon cooling, the infiltrant phase retained all or part of the dissolved iron which is precipitated upon back drawing with a tendency to harden the infiltrated phase.

In any case, the cuprous and ferrous phases of the body or blade are superficially alloyed and firmly bonded together during the above heat treatments.

While the aluminum-bearing coating was only slightly alloyed with the surface metal of the body at a temperature above the transformation point of the ferrous skeleton, the solubility of aluminum in alpha iron (below the transformation point) is suddenly increased to about 33% and therefore the heating between 300 and 600 and preferably at 500 C. for about one hour effects alloying to desired extent between the aluminum-bearing coating and surface metal of the body.

The body or part (blade) thus coated can now be provided with a corrosion resistant outer surface, or can be pressure-shaped (straightenedcoined) in a die. In the latter case the aluminum layer of the coating acts like a lubricant during shaping in the die. In order to provide the corrosion resistant surface, the outer surface layer of the aluminum coating may be oxidized, e. g. by an electrolytic treatment, by exposure to an oxidizing atmosphere at suitable temperature below its melting point for a suficient period of time, Or otherwise. A casing of refractory stable metal oxide or mixture of oxides, such as of aluminum, titanium, manganese and/or zirconium, can also be deposited on the aluminum coating.

A body or part of the nature and type herein concerned and produced in the manner hereinbefore described by way of a preferred example, having a ferrous matrix (skeleton) of a density of about 82 to 85% infiltrated with a copperiron-manganese alloy, exhibited after infiltration and dipping into the molten aluminum alloy 9. yield strength of 74,000 p. s. i., a tensile strength of 86,000 p. s. i., an elongation of 8.7% and a Rockwell B hardness of upon reheating at 500 C. for one hour the yield strength was increased to 85,000 p. s. i., the tensile strength to 92,400 p. s. i. and the hardness to 98 Rockwell B, whereas the elongation was only slightly reduced to 7%.

It should be understood that the invention is not limited to any exempliflcation hereinbefore 7 described but is to be derived in its broadest aspects from the appended claims.

What I claim is:

1. The method of producing an aluminumcoated article having a ferrous skeleton infiltrated with a cuprous infiltrant and containing an appreciable amount of carbon up to 1.7%, comprising the steps of heating the infiltrated article at a temperature between the melting point of the infiltrant and about 1250 C. to dissolve constituents of the ferrous skeleton in the molten infiltrant and to diffuse constituents of the infiltrant into the ferrous skeleton, cooling the article at a rate to retain diffused constituents of the infiltrant in the ferrous skeleton in excess of the amount normally soluble therein at room temperature and to retain dissolved constituents of the skeleton in the cuprous infiltrant in excess of the amount normally soluble therein at room temperature, pressure-shaping the article, heating the article to a temperature between about 735 C. and about 850 C., dipping the article into a molten aluminum-bearing metal maintained at a temperature approximately the same as that of the heated article to form a coating thereof on the article, removing the coated article from the molten metal, cooling it to a temperature below the freezing point of the coating, and heat treating the coated article at a temperature between about 300 C. and 600 C. to precipitate excess constituents of the infiltrant in the skeleton and of the skeleton in the infiltrant and to alloy the coating and the surface metals of the article.

2. The method of producing a composite ferrous metal article of high strength which comprises compacting and forming ferrous particles containing at most about 25% carbon into a shaped sintered porous skeleton having a sufficientiy low carbon content so that when said skeleton is quenched from a, raised temperature it exhibits a relatively high degree of softness and is readily given the desired shape, infiltrating said skeleton with a molten cuprous infiltrant having a melting temperature lower than 1250 C., maintaining the skeleton and infiltrant at a temperature in the range from about 1100 C. to about 1250 C. and above the melting temperature of the infiltrant for a substantial time to cause diffusion between the infiltrant and the skeleton and penetration of a substantial amount of constituents of the inflltrant substantially throughout the skeleton particles, thereafter cooling the infiltrated body from elevated temperature at a quenching rate to give said body relative softness so that it may be further formed into the desired shape, thereafter further forming said body into the desired shape, thereafter coating said body with molten aluminum-bearing metal maintained at a temperature in the range between about 735 to 850 C.. and thereafter treating the coated article at a temperature between about 300 C. and 600 C. to precipitate excess constituents of the infiltrant metal in the skeleton metal and of the skeleton metal in the infiltrant metal and thereby materially increase the strength of said article.

3. The method of producing a composite ferrous metal article of high strength which comprises compacting and forming ferrous particles containing at most about 0.25% carbon into a shaped sintered porous skeleton having a sufficiently low carbon content so that when said skeleton is quenched from an elevated temperature, it exhibits relative softness so that it may be readily given the desired final shape, subjecting said sintered skeleton to an additional forming operation for giving it the desired shape. infiltrating said shaped skeleton with a molten cuprous infiltrant having a melting temperature lower than 1250 C., maintaining the skeleton and infiltrant at a temperature in the range from about 1100 C. to about 1250 C. and above the melting temperature of the infiltrant for a substantial time to cause diffusion between the infiltrant and the skeleton and penetration of a substantial amount of constituents of the infiltrant substantially throughout the skeleton particles, thereafter cooling the infiltrated body from elevated temperature at a quenching rate to give said body relative softness so that it may be further formed into the desired shape,

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,501,887 Crapo July 15, 1924 1,966,130 Norton July 10, 1934 2,401,221 Bourne May 28, 1946 2,402,950 Culver et al. July 2, 1946 2,422,439 Schwarzkopf June 17, 1947 2,456,779 Goetzel Dec. 21, 1948 OTHER REFERENCES Wulff: Powder Metallurgy, published by American Society for Metals, Cleveland, Ohio, 1942, pages 29-569. 

1. THE METHOD OF PRODUCING AN ALUMINUMCOATED ARTICLE HAVING A FERROUS SKELETON INFILTRTATED WITH A CUPROUS INFILTRANT AND CONTAINING AN APPRECIABLE AMOUNT OF CARBON UP TO 1.7%, COMPRISING THE STEPS OF HEATING THE INFILTRATED ARTICLE AT A TEMPERATURE BETWEEN THE MELTING POINT OF THE INFILTRANT AND ABOUT 1250* C. TO DISSOLVE CONSTITUENTS OF THE FERROUS SKELETON IN THE MOLTEN INFILTRANT AND TO DIFFUSE CONSTITUENTS OF THE INFILTRANT INTO THE FERROUS SKELETON, COOLING THE ARTICLE AT A RATE TO RETAIN DIFFUSED CONSTITUENTS OF THE INFILTRANT IN THE FERROUS SKELETON IN EXCESS OF THE AMOUNT NORMALLY SOLUBLE THEREIN AT ROOM TEMPERATURE AND TO RETAIN DISSOLVED CONSTITUENTS OF THE SKELETON IN THE CUPROUS INFILTRANT IN EXCESS OF THE AMOUNT NORMALLY SOLUBLE THEREIN AT ROOM TEMPERATURE, PRESSURE-SHAPING THE ARTICLE, HEATING THE ARTICLE TO A TEMPERATURE BETWEEN ABOUT 735* C. AND ABOUT 850* C.,DIPPING THE ARTICLE INTO A MOLTEN ALUMINUM-BEATING METAL 